1. Field of the Invention
This invention relates to the field of escape mechanisms for aircraft.
2. Prior Art
Within the prior art a variety of systems and devices are known which functions as crew escape systems. During World War II, the original ejection seats were developed for use by the German Air Force. These were the original open ejection seats and following their development, two distinct lines of research ensued. The most direct development of the open ejection seat was an encapsulated seat in which the crew member was completely surrounded by protective structure and was first put into service with the General Dynamics B-58 aircraft. The encapsulated seat could function either as a shelter on land or used in water as a lifeboat. A variation of the encapsulated seat is the flyable ejection seat (AERCAB) and some developmental work on this concept is currently underway. A third variation is the escape capsule which is, in essence, a second aircraft inside the first.
A second developmental line occurs with the use of rocket extraction as opposed to open ejection seat types of escape. The concept of using a spin-stabilized suspended rocket for deceleration was originated at Stencel Aero-Engineering Corporation in the lat 1950's and this Firm developed a system to decelerate air dropped stores just prior to impact (G PADS, for ground proximity air deceleration system). In this device, the rocket provided a force to provide an upward acceleration, thereby reducing the descent velocity. The dynamics involved were essentially those of rigid bodies using inextensible lines. This system found utilization in the concept of crew escape devices in the sense that it was realized that by attaching the rocket pendant to a parachute-type torso harness, only the crew member need be pulled from the aircraft and such an extraction could be safely effected. It appears, however, that the analysis of this system failed to realize that the dynamic and aerodynamics of the pendant could dominate the performance of the system, with the rocket oscillating on the pendant resiliency to give wide excursions of line tension. Accordingly, the stiffness of the pendant seems to have been more or less arbitrarily determined, with the unfortunate result that the rocket oscillates on it at a frequency of about 10 Hz. If damping in the line is small, the line may go slack for the first few cycles of this oscillation.
As the crew member emerges into the wind blast, a drag force acts upon those parts of his body which are exposed to the blast. Until his abdomen clears the windshield, these drag forces are all above his center of gravity so that they tend to pitch him backwards. For this system to be effective, the extraction pendant must pull the crew member forward as well as up.
A typical rocket extraction system at high speeds experiences the problem of rearward pitching motion partly because the rocket trajectory is not inclined sufficiently forward but also because the thick (typically 11 mm. diameter) pendants are blown back to a quasi-catenary shape. Even though the rocket may be pulling upward and forward at the top end of this catenary, the crew member at the other end is being pulled upward and back. It can be shown that this backward force may be as high as 900 pounds during a 600 knot escape, even though the rocket is directly above the escapee. Accordingly, the backward flip seen with such systems in high speed escapes is due partly to aerodynamic loads on the man and partly to the aerodynamic forces on the pendant. It is known that a human can withstand surprisingly high aerodynamic pressures when moving head first through a fluid, but that man is readily injured or killed if he moves feet first.